Along the spine of an animal is usually best for belts, belly leather is more supple, and prone to stretching, so better for things that will not be affected by the flexibility, and shoulders are different to the hindquarters, so yes, the location on the hide where your leather comes from should be considered when choosing leather for a specific task.
Kangaroo leather is usually about1.0 - 1.5mm, but I have seen 5.0+mm kangaroo leather. It was just a small patch where the tail joined to the back on a hide I tanned myself about 35years ago. Overall the hide was quite flexible, but the thicker section was like plywood.